Without protection devices, exposure to high intensity noises may lead to temporary and/or permanent damage or hearing loss. Conventional passive hearing protection devices, such as inexpensive foam ear plugs, have been widely used since the end of World War II. These passive hearing protection devices attenuate the incident acoustic energy across all frequencies of sound at a fixed ratio.
Active noise reduction or active noise control devices incorporate noise-canceling circuitry to reduce unwanted sound by the addition of a second sound designed to cancel the first. These active hearing protection devices sense incoming acoustic waves, generate new acoustic waves with the same amplitude as—but the inverted phase of—the original acoustic waves, and emit the new acoustic waves for destructive interference with the original acoustic waves.